What should my hull speed be?

SteveK said:

Sling weight and displacement weight are not the same. My GB 36 was rated 26K displacement and weighed 34K in the sling

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Perhaps semantics are at play, but are you suggesting that your 34,000 pound boat displaces only 26,000 pounds of water? As a matter of physics, aren't the two necessarily identical? If the distinction between weight and displacement is semantic, please explain.

MYTraveler said:

Perhaps semantics are at play, but are you suggesting that your 34,000 pound boat displaces only 26,000 pounds of water? As a matter of physics, aren't the two necessarily identical? If the distinction between weight and displacement is semantic, please explain.

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No, I was replying to another post that I should have quoted, but this is off the rails like so many threads. look back for the 31,000 lb boat in the sling and everyone saying your boat is not 31,000.
I was trying to make the point of new boat quoted weight and then actual in sling weight being different. I did so poorly.
Why would someone question the in sling weight, because they are thinking the stock weight.
My GB36 is 11.1t or 24,250 lbs, yet the lift weighed at 34k (I know 10K is a lot)
So weight has to be understood as bareboat and as loaded.

I agree. Boat specs are boat specs. They generally post one spec for displacement.

But they offer 1 or 2 engines of different sizes. Different generators. Different tanks.

The displacement spec on my boat is 34,000 lbs. It shows 42,000 in the sling.

SteveK said:

No, I was replying to another post that I should have quoted, but this is off the rails like so many threads. look back for the 31,000 lb boat in the sling and everyone saying your boat is not 31,000.
I was trying to make the point of new boat quoted weight and then actual in sling weight being different. I did so poorly.
Why would someone question the in sling weight, because they are thinking the stock weight.
My GB36 is 11.1t or 24,250 lbs, yet the lift weighed at 34k (I know 10K is a lot)
So weight has to be understood as bareboat and as loaded.

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We own a DeFever 44. Its advertised weight is 44,000#. In the slings, with a half load of fuel and water, it weighs 56,000#. A full load would add another 3,500#.

If they only publish a single weight for a given boat, it's typically dry weight for the lightest option combination (and even then, not always terribly accurate). Any additional options, fuel and water in the tanks, stuff on board, etc. all add to that weight number.



I haven't actually weighed my boat, but it should weigh in a little over 27,000 lbs (with dinghy in the davits, full tanks, typical load of stuff, no people on board). That's assuming that the dry weight spec of 21,600 isn't too optimistic (I fudged it up to 22,000 when I estimated loaded weight). They were nice enough to list 2 dry weights though (gas vs diesel engines). The diesels (which almost nobody bought in this boat) bumped the dry weight to 23,600.

Travel lift scales, unless calibrated, are not too reliable. Their main purpose is to insure lift and slings are not overloaded, with safety factors plugged in. As noted, theoretical hull speed is partially a matter of WLL. But hull shape and weight play a big part too.

sunchaser said:

Travel lift scales, unless calibrated, are not too reliable. Their main purpose is to insure lift and slings are not overloaded, with safety factors plugged in. As noted, theoretical hull speed is partially a matter of WLL. But hull shape and weight play a big part too.

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Based on the typical maintenance on the typical travel lift, plus in my experience the sling weight gauges were mostly broken/inaccurate....

then again I never worked at/frequented top notch marinas.

Also the lift 'scale' may include all suspended weight, like spreader bars and slings.

Boat manufacturers weigh is usually way low. Maybe they calculate the weight or weight one of the first boats of a model, but then they start adding options and the weight goes up. And the scales on travel lifts are notoriously wrong…

I don't think my numbers were too far off. The scale showed an 8000 pound difference.

I looked up the weight of 800 gallons of diesel and 200 gallons of water and it was 7000 pounds.

On a new diesel powered boat, we'd aim for 50-75 rpm over the rated rpm at WOT. That's with a "light load", strategy being that with a full load, rpm's would be at manufacturer's max rated rpm at wide open throttle. Cummins, for one, would not approve an installation if this wasn't followed during sea-trials. You're a used boat at full load, so in an ideal world you should be propped to meet rated rpm. Things change as a boat accumulates hours of course so you have some latitude. This isn't a ski boat, you don't adjust the prop to get a certain speed. It is what it is...

Get Dave Gerr's "Propeller Handbook" for an in-depth discussion of props. It's incredibly informative - it was on my desk when I was spec'ing out props.

markpierce said:

You will increase efficiency (reducing fuel consumption by 50% or more) moving one knot below hull speed. At hull speed, my fuel consumption goes from 4 gallons an hour to 1.7 at one knot below hull speed.

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So it is reasonable to assume then, based on what I have been reading to this point, that my 48 ft ( 43 ft LWL ? ) should economically cruise at about 8 knots? I am powered by a detroit 671 rated at 165 hp. THe boat tips the scales at 41,200 pounds.

rslifkin said:

Good point. A photo tach is fairly cheap to buy and will allow you to calibrate the tach and make sure it's accurate.

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Even cheaper is a permanently mounted digital rpm with magnetic sensor and readout. Less than $20. Easy install.

I have installed on both engine and shaft. Takes the guesswork out.

Disregarding the possible faulty Tach, i think the OP's speed data is fine. Very comparable to my Monk designed 37, just below 7 knots is where the wake size increases and diminishing returns start.

Speeds beyond that are pay to play.

Tinman said:

So it is reasonable to assume then, based on what I have been reading to this point, that my 48 ft ( 43 ft LWL ? ) should economically cruise at about 8 knots? I am powered by a detroit 671 rated at 165 hp. THe boat tips the scales at 41,200 pounds.

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Your theoretical hull speed (and remember, it is just "theoretical") is a bit under 8.8 knots. So 8 knots should be a reasonable cruising speed. Of course, slower will be more economical.

The displacement of your boat has no bearing on its theoretical hull speed. What the displacement DOES have a bearing on is how many horsepower are required to reach that speed.

Blackinblue wrote;
“Also, probably better to be a little under propped than over propped”

rslifkin wrote;
“There's no real harm in being under propped, but if you see 3000 rpm the tach is wrong.”

You can only harm your engine from being overpropped. That’s a fact unless you run at a lower rpm. Lower than rated rpm. Like 2300rpm w an FL.

IMO 100rpm above rated is ideal.

Rated rpm is NOT a never exceed engine speed.

Hull Speed

Typically hs is 1.34 x the square root of the WLL.
But it’s not cast in stone.

For example;
OAL is not a factor. It’s strictly water line length.
Somewhat harder to understand is that a full shaped bow or stern (or both) has a significant effect on speed (or lack of) when considering boat speed and the formula.

A FD boat w a full bow (under water) will be ultimately faster but require more power. A boat w a very pointy forefoot (bow) will act (hs wise) differently. Her power requirement to run a knot below hs will be less.
A boat’s normal loaded WLL should be considered re hs matters.
Some boats have (mostly sailboats) a long overhang aft and will have a considerably variable WLL depending on boat load.

There’s lots of variables.
But generally a FD boat should usually run (like in cruising mode) quite close to one knot below hs. A Willard 30 (FD) is a good example and mine ran at about 6.15 knots w a WOT speed of a bit (very small bit) over hs … as in 7 plus a tad knots. Rpm was 2300 cruise and 3100 rpm (rated at 3000) at WOT.

Capt Dan said:

Even cheaper is a permanently mounted digital rpm with magnetic sensor and readout. Less than $20. Easy install.

I have installed on both engine and shaft. Takes the guesswork out.

Disregarding the possible faulty Tach, i think the OP's speed data is fine. Very comparable to my Monk designed 37, just below 7 knots is where the wake size increases and diminishing returns start.

Speeds beyond that are pay to play.

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Thanks for the input. I had not considered using a digital tach, but it sounds like a great idea. For the sake of calculating fuel consumption, I will go with 2 gph at 7 knots.

In post #46
I wrote;
“ A FD boat w a full bow (under water) will be ultimately faster but require more power. A boat w a very pointy forefoot (bow) will act (hs wise) differently. Her power requirement to run a knot below hs will be less.””

Most musta thought I was nuts .. not yet haha.
A FD boat operates in the trough of a wave created by the boat itself.
With a fuller bow the bow wave will be created sooner and w a full stern (like my Willard) will be a bit further aft and the stern wave further aft. With the two waves further apart the wave will be longer. A longer wave will move faster carrying the boat along w it.

The opposite is a sailboat w both ends fine. The wave will be shorter w both ende fine. The boat will be slower and w considerably less power requirement. They cal this hull a diamond shaped hull. The slowest and most easily driven.

Hope that explains a boat w full/blunt ends will be faster. It’s very rare in the US but a boat of this description is common in the Netherlands. I think they call them barges.

Had a 210 Cummins in my 38ft Marine Trader, only ran at 1600rpm for average 7.3 knots fuel burn about 2 1/4 gph.

I love theories. When the builders develop that curve of RPM, speed, fuel consumption curve, they are an empty boat and we all aware, no one cruises in a empty boat.

Tinman said:

So it is reasonable to assume then, based on what I have been reading to this point, that my 48 ft ( 43 ft LWL ? ) should economically cruise at about 8 knots? I am powered by a detroit 671 rated at 165 hp. THe boat tips the scales at 41,200 pounds.

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8-kts is right at 1.2 S/L for 43-foot waterline. 1.2 S/L is a good speed for most boats in this class as it's usually enough to get the engine(s) well into their comfortable operational temps. In my opinion, 1.34 S/L, the generally accepted 'hull speed' is the speed at which the boat starts transitioning ----- and efficiency starts dropping like a rock as you can hear/feel the engine(s) and prop(s) starting to load-up. Stay at 7.5-8.0 kts and you won't lose much time and your wallet will thank you.

Just my opinion.....

Peter